Enthalpy of formation of cubic yttria-stabilized zirconia

Oxide melt solution calorimetric measurements were made to determine the enthalpy of formation of cubic-yttria-stabilized zirconia (c-YSZ) with respect to the oxides m-ZrO2 and C-type YO1.5. The enthalpy of formation can be fit either by a quadratic equation or by two straight line segments about the minimum near x = 0.40. The quadratic fit gives a strongly negative interaction parameter, Omega = -93.7 +/- 12.0 kJ/mol, but does not imply regular solution behavior because of extensive short-range order. In this fit, the enthalpy of transition of m-ZrO2 to c-ZrO2, 9.7 +/- 1.1 kJ/mol, is in reasonable agreement with earlier estimates and that of C-type to cubic fluorite YO1.5, 24.3 +/- 14.4 kJ/mol, is consistent with an essentially random distribution of oxide ions and anion vacancies in the high-temperature fluorite phase. The two straight-line segments give 6.1 +/- 0.6 kJ/mol and 5.5 +/- 2.5 kJ/mol for these transitions, respectively. The latter value would imply strong short-range order in cubic fluorite YO1.5. Clearly more complex solution thermodynamic descriptions need to be developed. The enthalpy of transition from the disordered c-YSZ phase to the ordered delta-phase at 25 degreesC was also measured and was 0.4 +/- 1.6 kJ/mol. No energetic difference between the disordered c-YSZ phase and the ordered delta-phase underscores the importance of short-range order in c-YSZ. Enthalpy data were combined with Gibbs free energy data to calculate entropies of mixing. Using the quadratic fit, negative excess entropy of mixing in the cubic solid solution, relative to a system with maximum randomness on cation and anion sublattices, was found and was another indication of extensive short-range order in c-YSZ.